DESCRIPTION: Presbyopia, the age-related loss of the ability to accommodate, is the most common ocular affliction, affecting every human over the age of 45 years. Its pathophysiology is poorly understood. In the rhesus monkey, the only known animal model for the human condition, the ciliary muscle loses with age its configurational responses to cholinomimetic drugs or central stimulation. Although some evidence suggests that this loss of mobility may be due to restriction by a progressively inelastic posterior attachment, it is not clear that the muscle's contractile machinery continues to function normally, nor what roles the lens and zonules might play. The project will determine, in rhesus monkeys encompassing the entire species lifespan: 1) the real- time dynamics of ciliary muscle, lenticular and zonular movement in response to midbrain stimulation in living, surgically aniridic animals, by digital image analysis of Scheimpflug and goniovideographic recordings; and 2) the effect of extra- and intra-capsular lens extraction, posterior capsulotomy and complete posterior capsulectomy following extracapsular lens extraction, and regional zonulolysis on these real-time dynamics. These studies will generate new information about the pathophysiology of presbyopia in the rhesus monkey, the only known animal model for the human condition.